Solution and process for the activation of nonconductive area for electroless process

ABSTRACT

The present invention discloses a novel activator system for electroless metallization deposition, particularly activators that may be free of tin and surfactants. Activators of the invention are preferably employed for electroless copper deposition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No. 17/184,119, filed on Feb. 24, 2021, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Technical Field of the Invention

The present invention relates generally to the field of plating activator, particularly novel activator system for electroless metallization deposition. The activator system of the invention is preferably employed for electroless copper plating.

Background

In general, a printed circuit board (PCB) is a component in which electric wirings are integrated to allow various devices to be populated therein or to be electrically connected to one another. Technological developments have led to an increase in production of PCBs having various forms and functions. Demand for such PCBs has been increasing with a growth of industries using the PCBs and relating to, for example, home appliances, communication devices, semiconductor equipment, industrial machinery, and electrical control of vehicles.

Electroless metal deposition has a wide range of applications, including for use in the manufacture of printed circuit boards and non-conductors such as decorative and engineering plastic substrates. Printed circuit boards include laminated non-conductive dielectric substrates that rely on drilled and plated through-holes to form connections between opposite sides or inner-layers of the boards. Electroless plating is well known for preparing metallic coatings on surfaces. Electroless plating of dielectric surfaces require the prior deposition of a activator. A common method used to catalyze or activate laminated non-conductive dielectric substrates, prior to electroless plating, is to treat the substrate with an aqueous tin-palladium colloid in an acidic chloride medium. The colloid includes a metallic palladium core surrounded by a stabilizing layer of tin (II) ion complexes, e.g., SnCl3⁻, which act as surface stabilizing groups to avoid agglomeration of the colloids in suspension.

In the activation process the palladium-based colloid is adsorbed onto an insulating substrate such as epoxy or polyimide to activate electroless copper deposition. Theoretically, the activator particles play roles as carriers in the path of electron transfer from reducing agents to metal ions in the plating bath. Although the performance of electroless copper plating is influenced by many factors, such as composition of the deposition solution and choice of ligand, the activation step is the key factor for controlling the rate and mechanism of electroless deposition. Tin/palladium colloid has been commercially used as activator for electroless metal deposition for decades. However, its palladium's sensitivity to air and high costly leave room for improvement. In addition, the residual palladium adsorbed on the resin surface must be removed to prevent possible short circuits between the two copper wires, thus increasing the cost of the overall manufacturing process.

Considerable effort has been made to find new and better activators. For example, because of the high cost of palladium, much of the effort has been directed toward the development of a non-noble activator, particularly towards the development of a colloidal copper activator. However, such activators have not been shown to be sufficiently active or reliable enough for through-hole plating. Furthermore, these activators typically become progressively less active upon standing and the change in activity renders such activator unreliable and impractical for commercial use.

SUMMARY OF INVENTION

Accordingly, one aspect of the invention relates to a composition for depositing an electroless plating activator on a substrate comprising one or more metal ions and one or more organic acid; wherein the organic acid has at least one carboxylic group and at least one hydroxyl group.

In some embodiments, the organic acid is dicarboxyl acid terminated and has a formula:

HOOC—R₁—COOH  (I)

-   -   wherein R₁ is chosen from linear or branched, substituted or         unsubstituted (C₁-C₆) alcohol.

In some embodiments, the organic acid is selected from the group consisting of tartaric acid, citric acid, malic acid, and 2,2-Bis(hydroxymethyl)malonic acid.

In some preferred embodiments, the organic acid is tartaric acid or malic acid.

In some embodiments, the organic acid has a formula:

R₂—COOH  (II)

-   -   wherein R₂ is chosen from linear or branched, substituted or         unsubstituted (C₁-C₆) alcohol.

In some embodiments, the organic acid is selected from the group consisting of glyceric acid, glycolic acid, and lactic acid.

In some preferred embodiments, the organic acid is glyceric acid.

In some embodiments, the metal ion is selected from the group consisting of palladium, copper, silver, gold, platinum, iridium, aluminum, cobalt and nickel ions.

In some preferred embodiments, the metal ion is copper.

In some embodiments, the pH of the composition is greater than 9.

Another aspect of the present invention relates to a method for depositing an electroless plating activator on a substrate, comprises: (a) applying the said composition to the substrate; (b) applying a reducing agent to the substrate.

In some preferred embodiments, the reducing agent is DMAB or NaBH₄.

A further aspect of the present invention relates to a method for forming an electroless copper plating film on a substrate, comprising: (a) depositing the said electroless plating activator on the substrate; (b) electrolessly plating copper on the substrate.

In some embodiments, the electrolessly plating step uses a plating bath comprising: tartrate, copper ions, formaldehyde, and 2,2-dipyridine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the proposed chelation models for complexing agents having one —COOH and one —OH with metal ions, where R stands for a linker.

FIG. 2 depicts the proposed chelation models for complexing agents having two —COOH and one —OH with metal ions, where R stands for a linker.

FIG. 3 shows the proposed binding mode of tartaric acid with metal ions.

FIGS. 4A and 4B show the plating coverage of the through-holes of (A) Example 1 and (B) Comparative Example 1 by backlight test.

FIGS. 5A and 5B show the plating coverage of the through-holes of (A) Example 2 and (B) Comparative Example 2 by backlight test.

FIGS. 6A and 6B show the plating coverage of the through-holes of (A) Example 3 and (B) Comparative Example 3 by backlight test.

FIG. 7. shows the plating coverage of the through-holes of Example 4 by backlight test.

DETAILED DESCRIPTION OF THE INVENTION

All technical and scientific terms used herein, unless specifically stated otherwise, have the same meaning as commonly understood by one of skill in the art. In the event of a conflict in meaning, this specification shall prevail.

The terms “printed circuit board” and “printed wiring board” are used interchangeably throughout this specification. The terms “plating” and “deposition” are used interchangeably throughout this specification. All amounts are percent by weight, unless otherwise noted. All numerical ranges are inclusive and combinable in any order except where it is logical that such numerical ranges are constrained to add up to 100%.

In the following, the technical contents, features, and achievements of the present invention will be described with specific implementation examples and can be implemented accordingly. However, the scope of protection of the present invention is not limited thereto.

Typically, when the substrate to be metal plated is a dielectric material such as on the surface of a printed circuit board or on the walls of through-holes, the boards are degreased followed by desmearing the through-hole walls. Preferably, the substrate to be plated is a metal-clad substrate with a dielectric material and a plurality of through-holes. The substrates are cleaned and degreased first, and followed by desmearing the through-hole walls. Typically prepping or softening the dielectric or desmearing of the through-holes is conducted by a sweller solvent.

After the desmearing a conditioner may be applied. Examples of conditioners in this disclosure contain monoethanolamine, one or more quaternary amines, one or more non-ionic surfactants, one or more conditioning polymers, and pH adjusters. In some embodiments, such conditioners contain 5-20 g/L of monoethanolamine, 0.1-15 g/L of triethanolamine, 0.1-10 g/L of triton X-100 (Dow Inc.), 1-5 g/L of basotronic PVI (BASF), and sodium hydroxide for adjusting pH. Optionally, the substrate and through-holes are then rinsed with water.

Conditioning may be followed by microetching. microetching is designed to provide a micro-roughened metal surface on exposed metal (e.g. inner-layers and surface etch) to enhance subsequent adhesion of deposited electroless and later electroplate. Etching cleaners include 50-150 g/L of sodium persulfate and 10-30 ml/L of sulfuric acid (98%). The microetched substrate is then rinsed with water for the following processes.

A pre-dip may then be applied to the microetched substrate and through-holes. The pre-dip helps to stabilize the activator bath pH and clean the metal surface. Preferably the pre-dip is used because it helps improve interconnect defects reliability. Conventional pre-dip aqueous solutions of inorganic or organic acids with a pH range typically from 3-5 may be used.

However, in some embodiments of the present disclosure, activation is performed in an alkaline condition, so the pH of the pre-dip solution may also be greater than 7. The pre-dip solution can be sodium hydroxide, sulfuric acid, boric acid, or a combination thereof to adjust to a desired pH. In some embodiments of the present disclosure, the activation can then be performed in an alkaline condition with a composition comprising complexing agents and metal salts.

The activator composition including metal ions and organic acids having one or more carboxylic group and one or more group hydroxyl group forms a stable aqueous solution of complexes which may be used to catalyze electroless metal deposition. Activator compositions of the invention are preferably alkaline. It is believed that maintaining a substantially alkaline pH promoted formation of a complex of composition components, which in turn promotes enhanced properties of the composition.

The organic acids having carboxylic group and hydroxyl group as a complexing agent provide adequate chelating power to metal ions, especially to copper ions. The proposed chelation models for such complexing agents with metal ions are shown in FIG. 1 (one —COOH, one —OH) and FIG. 2 (two —COOHs, one —OH), where R stands for a linker. The tartaric acid in this disclosure exemplifies the binding modes of acids having two carboxylic groups and two hydroxyl groups (shown in FIG. 3).

Metal ions may be provided by conventional metal salts. Typically, such metal salts are included in the activator solutions to provide metal ions in amounts of 20 ppm to 5000 ppm, preferably from 200 ppm to 1500 ppm. Metal ions include, but are not limited to: silver, gold, platinum, palladium, copper, cobalt and nickel ions. Preferably, the metal ions are chosen from copper and palladium ions. Metal ions may be provided by using conventional water-soluble metal salts which are well known in the art and may be found in the literature.

If the activator is an ionic activator where the metal ions have not yet been reduced to their metal state, a reducing solution is then applied to the substrate to reduce the metal ions of the activator to metal. The reducing solution may be applied by immersing the substrate into the reducing solution or spraying the reducing solution on the substrate. The reduction can then be performed in a reducing solution comprising 1-25 g/L of Dimethylamine Borane (DMAB). In some embodiments of the present disclosure, a reducing solution comprising NaBH₄ is also be used. Optionally, the activated substrate and through-holes are rinsed with water. It is noted that in some examples of the present disclosure, rinsing is not allowed at this stage

The substrate and walls of the through-holes are then plated with metal, such as copper, copper alloy, nickel or nickel alloy with an electroless bath. Preferably copper is plated on the walls of the through-holes. Plating times and temperatures may be conventional. The substrate may be immersed in the electroless plating bath or the electroless bath may be sprayed onto the substrate. Typically, plating may be done for 5 seconds to 30 minutes; however, plating times may vary depending on the thickness of the metal on the substrate.

The performance of activator system for catalytic electroless plating was assessed by examining the plating coverage of the walls of the through-holes. Each substrate is sectioned laterally to expose the copper plated walls of the through-holes. The plating coverage was determined by the amount of light that was observed under the microscope. If no light was observed the section was completely black and was rated 5 on the backlight scale indicating complete copper coverage of the through-hole wall. If light passed through the entire section without any dark areas, this indicated that there was very little to no metal is plated on the wall and the section was rated 0. If sections had some dark regions as well as light regions, they were rated between 0 and 5.

The following method can be used when electroless plating is performed on a non-conductive substrate using the activator aqueous solution of the present invention. The electroless copper plating is taken as an example. The following examples are not intended to limit the scope of the invention but to further illustrate the invention.

Desmear/De-Etching Process

A Desmear/de-etching process comprising: Composition Time Temp. A1 Sweller 5 min 60° C. Sweller PC* 350 ml/L NaOH 3 g/L A2 Rinse 1 min 20° C. A3 Permanganate etching solution 10 min 75° C. KMnO₄ 60 g/L NaOH 45 g/L A4 Rinse 1 min 20° C. A5 Reducing solution 5 min 40° C. Neutralizer PFH* 100 ml/L H₂SO₄ (98%) 40 ml/L A6 Rinse 1 min 20° C. *provided by Jetchem Co.

The substrate is degreased by washing with alkaline sweller solution. After rinsing with water, it is smear-microetched by alkaline permanganate solution followed by rinsing with water again and terminated by immersing in reducing solution comprising neutralizer and acid.

The processes after the desmear/de-etching are further described as follows.

Example 1

Composition Time Temp. 1. Conditioner 10 min 60° C. Monoethanolamine 10 g/L Triethanolamine 5 g/L Triton X-100 1 g/L Basontronic PVI 1 g/L 2. Rinse 1 min 20° C. 3. Etch cleaner 1 min 25° C. Sodium Persulfate 100 g/L H₂SO₄ (98%) 20 ml/L 4. Rinse 1 min 20° C. 5. Pre-immersion 5 min 25° C. NaOH solution pH = 9.0 6. Activator 10 min 40° C. Tartaric acid 1.44 g/L Pd ion 0.2 g/L  pH = 12.1 (adjust by NaOH solution) 7. Rinse 1 min 20° C. 8. Reducing 2 min 40° C. DMAB 6 g/L pH = 9.5 (adjust by NaOH solution) 9. Rinse 1 min 20° C. 10. Electroless plating 8 min 33° C. Electroless Cu MC* 11. Rinse 1 min 20° C. *provided by Jetchem Co.

In this example, the microetched substrates are immersed in a pre-dip solution containing NaOH for the following alkaline activation. The composition for activation comprises 1.44 g/L of tartaric acid and 0.2 g/L of palladium ion, and the pH is adjusted to 12.1. Activation is allowed to proceed for 10 minutes at 40° C., then the activated substrates are rinsed with water.

The reducing solution comprises 6 g/L of DMAB, and pH is adjusted to 9.5 using 1.0 N NaOH. Reduction is allowed to proceed for 2 minutes at 40° C., then the activated substrates are rinsed with water.

The substrates are immersed in the electroless Cu MC plating bath (Jetchem Co.) at 33° C. for 8 minutes. The backlight test results are shown in FIG. 4A.

Comparative Example 1

Composition Time Temp. 1-4.  See Example 1 5. Pre-immersion 5 min 25° C. H₂SO₄ solution pH = 2.3 6. Activator 10 min 40° C. Tartaric acid 1.44 g/L Pd  0.2 g/L pH = 1.3 (adjust by H₂SO₄ solution) 7-11. See Example 1

In this comparative example, except for the following conditions, the rest of the process is the same as in Example 1. The pre-dip solution contains 1.0 N H₂SO₄, and pH=2.3. The composition for activation comprises 1.44 g/L of tartaric acid and 0.2 g/L of palladium ion, and the pH is adjusted to 1.3. The backlight test results are shown in FIG. 4B.

The activator composition containing tartaric acid has been proven to be applicable between pH 12.1 and 1.3 according to Example 1 and its comparative example. However, the backlight test shows that the activator system performs better under alkaline condition than under acidic condition, with scores of 4.75 and 4.25, respectively. The deprotonation of the carboxylic acid promotes chelation and thus stabilizes the metal ion, allowing these acids to act as a mediator in the activation process.

Example 2

Composition Time Temp. 1-4.  See Example 1 5. Pre-immersion 5 min 25° C. H₃BO₃ solution 5.0 g/L pH = 9.0  (adjust by NaOH solution) 6. Activator 10 min 40° C. Malic acid 12.0 g/L Pd 0.2 g/L pH = 12.6 (adjust by NaOH solution) 7-11. See Example 1

In this example, the performance of complexing agent malic acid in activation are examined. The microetched substrates are immersed in a pre-dip solution containing 5.0 g/L of boric acid, pH=9.0, for 5 minutes at 25° C. The activation is performed in a composition comprising 12.0 g/L of malic acid and 0.2 g/L of palladium ion, and the pH is adjusted to 12.6 for 10 minutes at 40° C.

The reduction is performed in a reducing solution comprising 6 g/L of DMAB, and pH is adjusted to 9.5 for 2 minutes at 40° C. The substrates are plated with metal by immersing in the electroless Cu MC plating bath (Jetchem Co.) at 33° C. for 8 minutes. The backlight test results are shown in FIG. 5A.

Comparative Example 2

Composition Time Temp. 1-4.  See Example 1 5. Pre-immersion 5 min 25° C. H₃BO₃ solution 5.0 g/L pH = 2.3 (adjust by H₂SO₄ solution) 6. Activator 10 min 40° C. Malic acid 12.0 g/L  Pd 0.2 g/L pH = 1.3 (adjust by H₂SO₄ solution) 7-11. See Example 1

In this comparative example, except for the following conditions, the rest of the process is the same as in Example 2. The pre-dip solution contains 5.0 g/L of boric acid, and pH=2.3. The composition for activation comprises 12 g/L of malic acid and 0.2 g/L of palladium ion, and the pH is adjusted to 1.3. The backlight test results are shown in FIG. 5B.

Malic acid, another example of an organic acid having at least one carboxyl group and at least one hydroxyl group, can also be applied to the activation process at a pH between 12.6 and 1.3. The scores of the backlight test under alkaline and acidic conditions are 4.75 and 3.25 respectively.

Example 3

Composition Time Temp. 1-5. See Example 1 6. Activator 10 min 40° C. Tartaric acid 5.0 g/L Cu 1.5 g/L pH = 12.0 (adjust by NaOH solution) 7. Rinse 1 min 20° C. 8. Reducing 2 min 40° C. DMAB 12 g/L pH = 9.5  (adjust by NaOH solution) 9. Electroless plating 15 min 33° C. Potassium 30 g/L sodium tartrate Cu 2.5 g/L Ni 0.5 g/L NaOH 10 g/L HCHO 4 g/L 2,2-dipyridine 60 mg/L 10.  Rinse 1 min 20° C.

In this example, copper ions are used instead of costly palladium ions in the activator system. The microetched substrates are immersed in a pre-dip solution containing NaOH, pH=9.0, for 5 minutes at 25° C. The activation is performed in a composition comprising 5.0 g/L of tartaric acid and 1.5 g/L of copper ion, and the pH is adjusted to 12.0 for 10 minutes at 40° C. The reduction is performed in a reducing solution comprising 12 g/L of DMAB, and pH is adjusted to 9.5 for 2 minutes at 40° C. Optionally, the activated substrate and through-holes are then rinsed with water.

Optionally, the activated substrate and through-holes are then rinsed with water. In some examples, however, electroless plating may be performed immediately without rinsing after reduction to avoid deactivation of the newly deposited copper. The substrates are plated with metal by immersing in the electroless plating bath comprising 30 g/L of potassium sodium tartrate, 2.5 g/L of copper sulfate, 0.5 g/L of nickel sulfate, 10 g/L of NaOH, 4 g/L of HCHO and 60 mg/L of 2,2′-dipyridine at 33° C. for 15 minutes. The backlight test results are shown in FIG. 6A.

Comparative Example 3

Composition Time Temp. 1-5. See Example 1 6. Activator 10 min 40° C. Tartaric acid 5.0 g/L Cu 1.5 g/L pH = 12.0 (adjust by NaOH solution) 7. Rinse 1 min 20° C. 8. Reducing 2 min 40° C. DMAB 12 g/L H₃BO₃ solution 20 g/L pH = 3.0  (adjust by H₂SO₄ solution) 9. Electroless plating 12 min 33° C. Potassium 30 g/L sodium tartrate Cu 2.5 g/L Ni 0.5 g/L NaOH 10 g/L HCHO 4 g/L 2,2-dipyridine 60 mg/L 10.  Rinse 1 min 20° C.

In this comparative example, except for the following conditions, the rest of the process is the same as in Example 3. The reducing solution comprises 12 g/L of DMAB, 20 g/L boric acid and pH is adjusted to 3.0 using 1.0 N H₂SO₄. The backlight test results are shown in FIG. 6B.

In Example 3 and its comparative example, copper ions are proved to be compatible with this activator system. The subsequent reduction step is also one of the factors that affect the performance of activation. The backlight scores of using alkaline or acidic reducing compositions are 4.75 and 3, respectively.

Example 4

Composition Time Temp. 1-5. See Example 1 6. Activator 5 min 40° C. _(DL)-Glyceric acid 5.0 g/L Cu 0.5 g/L pH = 12.0 (adjust by NaOH solution) 7. Rinse 1 min 20° C. 8. Reducing 2 min 40° C. NaBH₄ 2 g/L pH = 12.6 (adjust by NaOH solution) 9. Electroless plating 15 min 33° C. Potassium 30 g/L sodium tartrate Cu 2.5 g/L Ni 0.5 g/L NaOH 10 g/L HCHO 4 g/L 2,2-dipyridine 60 mg/L 10.  Rinse 1 min 20° C.

The table above shows another example of using copper ions instead of palladium ions in the activator system. The activation is performed in a composition comprising 5.0 g/L of glyceric acid and 0.5 g/L of copper ion, and the pH is adjusted to 12.0 for 10 minutes at 40° C. The reduction is performed in a reducing composition comprising 2 g/L of NaBH₄, and pH is adjusted to 12.6 for 2 minutes at 40° C.

The substrates are immediately plated with metal by immersing in the electroless plating bath comprising 30 g/L of potassium sodium tartrate, 2.5 g/L of copper sulfate, 0.5 g/L of nickel sulfate, 10 g/L of NaOH, 4 g/L of HCHO and 60 mg/L of 2,2′-dipyridine at 33° C. for 15 minutes. The backlight test score is 5 (shown in FIG. 7), indicating that glyceric acid and NaBH₄ can be used for activation and subsequent reduction, respectively.

The foregoing description is only some preferred embodiments of the present disclosure and is not intended to limit the scope of the present invention. Any changes and modifications in the stereochemistry, concentration, temperature, pH, reaction time and spirits mentioned in the scope of the patent application shall be included in the scope of the patent application for this work. 

1. A method for depositing an electroless plating activator on a substrate, comprising: (a) providing a substrate; (b) applying an activator composition to the substrate; (c) applying a reducing agent to the substrate; wherein the activator composition comprising at least one metal ion and at least one organic acid; wherein the organic acid has at least one carboxylic group and at least one hydroxyl group.
 2. The method of claim 1, wherein the organic acid is dicarboxyl acid terminated and has a formula: HOOC—R₁—COOH  (I) wherein R₁ is chosen from linear or branched, substituted or unsubstituted (C₁-C₆)alcohol.
 3. The method of claim 2, wherein the organic acid is selected from the group consisting of tartaric acid, citric acid, malic acid, and 2,2-Bis(hydroxymethyl)malonic acid.
 4. The method of claim 3, wherein the organic acid is tartaric acid or malic acid.
 5. The method of claim 1, wherein the organic acid has a formula: R₂—COOH  (II) wherein R₂ is chosen from linear or branched, substituted or unsubstituted (C₁-C₆)alcohol.
 6. The method of claim 5, wherein the organic acid is selected from the group consisting of glyceric acid, glycolic acid, and lactic acid.
 7. The method of claim 6, wherein the organic acid is glyceric acid.
 8. The method of claim 1, wherein the metal ion is selected from the group consisting of palladium, copper, silver, gold, platinum, iridium, aluminum, cobalt and nickel ions.
 9. The method of claim 8, wherein the metal ion is copper.
 10. The method of claim 1, wherein the pH of activator composition is greater than
 9. 11. The method of claim 1, wherein the reducing agent comprises DMAB or NaBH₄.
 12. A method for forming an electroless copper plating film on a substrate, comprising: (a) depositing an electroless plating activator on the substrate by the method of claim 1; (b) electrolessly plating copper on the substrate.
 13. The method of claim 12, wherein the step (b) uses a plating bath comprising: tartrate, copper ions, formaldehyde, and 2,2-dipyridine. 